1. Field of the Invention
The present invention relates to a light wavelength converting apparatus for converting light of a semiconductor laser or a laser diode (LD) into second harmonic wave light, and particularly relates to a light wavelength converting apparatus for emitting laser light, which is capable of high-speed modulation driving, and can be used as a light source for laser display, optical recording, optical measurement, etc.
2. Description of the Related Background Art
A variety of attempts for conversion of LD light into light at another wavelength have been made using a nonlinear optical material. According to such technology, it becomes possible to generate laser light in a wavelength range, such as a green range or an ultraviolet range, that is not yet put into practice with the LD. Such a light source can be expected to be usable as a light source for laser display or optical recording. Particularly, research and development have been widely conducted with respect to a harmonic generation (SHG) system in which fundamental-wave light is input into a nonlinear optical material to generate light whose wavelength is half of that of the fundamental-wave light (second harmonic wave light). In connection therewith, Japanese Patent Application Laid-Open No. 2002-43683 discloses a driving method which is used when SHG light is generated using fundamental-wave light emitted from a distributed Bragg reflection (DBR) semiconductor laser.
FIG. 8 illustrates the driving method. In FIG. 8, a DBR semiconductor laser 1 is comprised of a gain region 11, a phase region 12 and a DBR region 13 with a diffraction grating, and the laser 1 emits fundamental-wave light. Temperatures of the phase region 12 and the DBR region 13 are controlled by injecting current perpendicularly to a pn junction provided therein, or injecting current into a thin-film heater provided therein. The refractive index of a waveguide is changed by such a change in the temperature. Phase and reflectivity for the fundamental-wave light in the DBR semiconductor laser 1 are accordingly adjusted to vary oscillation wavelength.
The fundamental-wave light is input into an SHG device 2. The SHG device 2 converts the wavelength of the fundamental-wave light, and outputs SHG light. The SHG light is input into an optical detector 3. The optical detector 3 converts the SHG light into an electrical signal. A control portion 8 includes a tentative control parameter calculator 81, a current-ratio calculator 82, and a control parameter determiner 83. The control portion 8 outputs a phase control current for the phase region 12 and a DBR drive current for the DBR region 13, based on the electrical signal from the optical detector 3.
Operation of the apparatus illustrated in FIG. 8 will now be described. An LD drive current is supplied to the gain region 11 of the DBR semiconductor laser 1 to drive the laser 1. The fundamental-wave light is accordingly output from the laser 1. The wavelength of the fundamental-wave light is converted by the SHG device 2, and SHG light is emitted therefrom. The SHG light is converted into an electrical signal by the optical detector 3. From the electrical signal, the control portion 8 determines a control parameter for controlling the phase control current and the DBR drive current without occurrence of any mode hop. The DBR semiconductor laser 1 is driven pursuant to the control parameter.
With the SHG device 2, a wavelength range having a large conversion efficiency is limited. Accordingly, when the wavelength of the fundamental-wave light is changed by controlling the currents injected into the phase region 12 and the DBR region 13, a range of the SHG light capable of being monitored by the optical detector 3 is likely to be very narrow. It is hence difficult to estimate the control parameter with high precision.
Therefore, in the apparatus of FIG. 8, a width of a wavelength capable of being shifted without occurrence of any mode hop is small. Further, when the wavelength range of the SHG device 2 having a large conversion efficiency is shifted due to a change in the device temperature, the control parameter needs to be changed. However, the apparatus of FIG. 8 does not consider such a situation. In other words, it is assumed in the apparatus of FIG. 8 that a temperature stabilizing mechanism, such as a Peltier device, for stabilizing temperature of the SHG device 2 and the semiconductor laser 1 at a constant value is used. Power consumption is therefore likely to increase in the apparatus of FIG. 8.